virtualbox: Document VirtualBox OVA appliance image

Current SDK images requests to manually setup the Virtual Machine in
VirtualBox, as we're only provinding the disk image (.vdi file).

Support to generate a VirtualBox Appliance image (.ova file) that works
out of the box has been recently added to `apertis-image-recipes`.

Starting Apertis release version `v2022`, SDK images will be uploaded in
OVA format. Let's document that and still keep the previous `.vdi` disk
image setup for old versions reference.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>